Specified bandwidths of variable gain amplifiers (i.e., VGA) in serial bus applications are getting wider as data rates increase. For example, at 28 gigabits per second the bandwidth of the VGA is commonly specified to be at least 75% of a maximum data rate, which means a 21 gigahertz bandwidth. The wider bandwidths are challenging, especially when the VGA gain is high. A gain-bandwidth-product limit of semiconductor processes makes circuit implementation of the VGAs difficult. Traditional VGA designs aim to make the VGA gain as flat as possible within the specified bandwidth region. The VGAs should provide a flat gain across the entire spectrum, so in effect a received signal is scaled by a prescribed amount. In practice, the VGA bandwidth reduces as the gain increases due to the gain-bandwidth-product limit. As a result, the bandwidth of a conventional VGA is smallest at the highest gain setting.
Referring to FIG. 1, a diagram illustrating a normalized graph of a conventional unequalized channel pulse response 10 is shown. A single impulse generated by a transmitter has a normalized duration of a unit interval (i.e., 1 UI) or symbol interval, with a time zero corresponding to a current cursor 16. Samples 12-14 correspond to an inter-symbol interference (i.e., ISI) to future symbols and are called precursor ISI. Samples 18-20 correspond to the ISI of past symbols and are called postcursor ISI.
It would be desirable to implement an extended variable gain amplification bandwidth with high-frequency boost.